1. Field of the Invention
The present invention relates generally to satellite communication systems and, more particularly, relates to an apparatus and method of power control to compensate for the effects of varying uplink signal attenuation, whatever their source. In the following description the invention and its operation are presented on the basis of varying uplink attenuation due to rain fall.
2. Description of the Relevant Art
A satellite communication system generally includes a satellite and several ground stations or nodes. The system is a frequency division multiplexed system for providing signal paths between various nodes via the satellite. A signal path includes an uplink, which is a signal transmitted from one node to the satellite at a given frequency in the frequency multiplexed communication system, and a downlink, which is the uplink signal frequency translated, power amplified and retransmitted by the satellite to the nodes in the communication system. The satellite has a transponder on board for receiving the uplink signals and transforming these signals into the downlink signals. Given a nominal operating point of the satellite transponder, variation in output, or downlink, power of a signal are very nearly equal to the variation to the input, or uplink, power of that signal. Once the nominal operating point has been selected, the downlink power of a particular signal is equal to the uplink power of that particular signal multiplied by a constant G.sub.s. Thus, the downlink power is controlled by the uplink power. The transponder in the satellite is characterized by a fixed total power limitation, P.sub.s, on the power that may be transmitted into the various downlink signals in the frequency multiplexed system.
A design problem common to the above-described satellite communication system is the allocation of the limited power, P.sub.s, of the satellite transponder between the various downlink signals. The design goal of the satellite communication system is to transmit the maximum possible number of downlink signals at any given time. One particular problem in such a satellite communication system is the attenuation of the uplink and downlink signals due to rain or other atmospheric conditions. A rain condition at a given node attenuates the uplink signal transmitted by the node and the downlink signals received by the node.
The attenuation of the downlink signal may cause the power of the downlink signal to be decreased to a level where extraction of the information contained in the downlink signal is precluded. Thus, the information contained within the downlink signal may be lost to the nodes, thereby causing a serious disruption in the communication system. To prevent this disruption, the nominal operating input/output power conditions at the satellite are established to guarantee operations of the downlink up to some maximum attenuation level including rain fade.
On the other hand, the attenuation of the uplink signal from the node during a rain condition may also cause the power of the downlink signal to the nodes in the system to be decreased, below the minimal downlink power level described above. This decrease is due to the nature of the transponder on board the satellite. Since the power of the downlink signal is a multiple of the power of the uplink signal, the attenuation of the uplink signal will decrease the power of the downlink signal. This decrease in the power of the downlink signal can result in the inability of the various nodes to extract the information carried by the signal from the transmitting local node and relayed by the satellite to a receiving node.
Presently, a rain condition at a node frequently shuts down the node since the uplink signal cannot penetrate the rain. If the uplink signal power was fixed at a level sufficient to burn through the rain then, in the absence of rain, the resulting downlink power would consume a large fraction of the total transponder transmit power, P.sub.s.
Accordingly, a system that compensates for atmospheric attenuation while allocating power among the various signals in the frequency multiplexed satellite communication system to maximize the information handling capacity of the system is greatly needed.